SENDAI, Japan -- U.S. naval barges loaded with fresh water sped toward Japan's overheated nuclear plant to help workers who scrambled Saturday to stem a worrying rise in radioactivity and remove dangerously contaminated water from the facility.

Workers at the stricken Fukushima Dai-ichi plant have been pumping seawater in a frantic bid to stabilize reactors that went haywire after a tsunami knocked out the complex's crucial cooling system earlier this month.

Plant operator Tokyo Electric Power Co. is now rushing to inject the reactors with fresh water instead amid concerns about the corrosive nature of the salt in seawater, Hidehiko Nishiyama of Japan's Nuclear and Industry Safety Agency said at a briefing Saturday.

The switch was the latest tactic in frantic efforts to gain control of the nuclear power plant located near the coast, 140 miles (220 kilometers) northeast of Tokyo.

Low levels of radiation have been seeping from the plant since a magnitude-9 earthquake and the ensuing tsunami on March 11, forcing an evacuation of families living within 12 miles (20 kilometers) of the plant.

Elevated levels of radiation have been found in raw milk, sea water and 11 kinds of vegetables, including broccoli, cauliflower and turnips, prompting several countries to halt some food imports from Fukushima region.

Tap water in several areas of Japan, including Tokyo, also measured at levels two times higher than the government standard for infants, who are particularly vulnerable to cancer-causing radioactive iodine.

On Friday, nuclear safety officials revealed that they suspected a breach in one or more of the plant's units, possibly a crack or hole in the stainless steel chamber around a reactor core containing fuel rods or the concrete wall surrounding a pool where spent fuel rods are stored.

Suspicions were aroused when two workers suffered skin burns after unexpectedly encountering water that was 10,000 times more radioactive than levels normally found in the units, NISA said.

Such a breach could mean a much larger release of radioactive contaminants. The most likely consequence would be contamination of the groundwater, experts said.

Following the announcement, government officials urged families in a voluntary evacuation zone between 12 and 19 miles (20 to 30 kilometers) from the plant to leave on their own.

Radioactivity was on the rise in some units due to contaminated water, Nishiyama said Saturday.

"It is crucial to figure out how to remove contaminated water while allowing work to continue," he said, acknowledging that the discovery would set back delicate efforts to get the plant's cooling system operating again.

Workers have begun pumping radioactive water from one of the units, Masateru Araki, a TEPCO spokesman, said Saturday.

Plant officials and government regulators say they don't know the source of the radioactive water. It could have come from a leaking reactor core, connecting pipes or a spent fuel pool. Or it may be the result of overfilling the pools with emergency cooling water.

But a breach in the chamber surrounding the core reactor seemed "more likely," Nishiyama said.

Tests of seawater just outside one unit tested some 1,250 times higher than normal, probably from both airborne radiation released from the reactors and contaminated water leaking into the sea, he said, adding that there was no immediate health risk.

Such levels are unlikely to pose a direct health risk to humans because they would not be expected to drink seawater or swim near the plant in these circumstances. This can affect the immediate marine environment, but the vast expanse of the sea will help dilute high levels of radiation.

The switch to cooling reactors with freshwater was necessary because of fears salt and other contaminants in sea water were clogging up pipes and coating the surface of reactor vessels and fuel rods, hampering the cooling process, NISA said.

Defense Minister Yoshimi Kitazawa said late Friday that the U.S. government had made "an extremely urgent" request to switch to freshwater. He said the U.S. military was sending water to nearby Onahama Bay and would begin water injections early next week.

The Pacific Command confirmed Saturday that barges loaded with freshwater were dispatched to Fukushima.

Temperatures have stabilized in one unit, Unit 1, since the switch to freshwater, Nishiyama said Saturday.

The nuclear crisis has compounded the challenges faced by a nation already overwhelmed by the disaster and devastation wrought by the tsunami.

Japanese soldiers and U.S. Marines were still clearing away debris so they could keep searching for bodies and bury the dead. The official death toll stood at 10,151 Saturday, with more than 17,000 listed as missing, police said. The final death toll was expected to top 18,000.

Hundreds of thousands whose homes were destroyed still have no power, no hot meals and, in many cases, have not showered in two weeks.

"It's still like I'm in a dream," said Tomohiko Abe, a 45-year-old machinist who was in the devastated coastal town of Onagawa trying to salvage any belongings he could from his ruined car. "People say it's like a movie, but it's been worse than any movie I've ever seen."

Government spokesman Yukio Edano said Friday that people living in the voluntary evacuation zone were safe from radiation as long as they stayed indoors. But he advised residents to leave due to a shortages in supplies to the area.

"If the current situation is protracted and worsens, then we will not deny the possibility of (mandatory) evacuation," he said.

The U.S. has recommended that people stay 50 miles (80 kilometers) away from the plant.

One Fukushima government official said some commercial trucks were refusing to enter the area because of radiation fears, resulting in a shortage of goods.

"We are not ordering people to leave. But we have told residents that we will help you leave voluntarily," Takeshi Ishimoto said.